Hi, your request, considering the age and type of Siemens PBX, is a challenge!
I presume your system could be a Siemens Hicom 150E R2.2 or, eventually, a 150H V1 - formerly known as 150E R3.0 (either the case…your Hardware/Software system was available years before Siemens introduced, really many years ago, the “new” HiPath 3000 V1.2 - then upgraded to V3, V4, V5, V6, V7, V8 and V9 - family). The HiPath 3000 family, reached V9, was recently Phased Out in favour of the new and shiny over-licensed linux based half open half closed source Unify OpenScape Business V1 (a masked HiPath 3000 V10 with new Software and a truly re-engineered Mainboard).
In the past Siemens introuced a Media-Gateway sub-module called HXGS (Small)/HXGM (Medium) formerly known as Siemens Hicom Xpress@LAN respectively available for Hicom 150E OfficePoint/OfficeCom or OfficePro (basically an H.323/CorNet-IP <-> system media gateway), that card then was renamed into Siemens HiPath HG1500 (V1, V2 and V3) with an Hardware completely re-engineered (same Form Factor for HiPath 33x0/35x0 or HiPath 37x0) and a new Firmware and Software (with new Features and Services, like SIP support for Trunking and Devices when coupled with HiPath 3000 V6 and newer) both enhanced to cope with new HiPath 3000 features.
All this long preamble to say that walking the track of finding and using an integrated media gateway is a no go (due to your system age and its intrinsic Hardware/Software/Interoperability limits): you should upgrade your system to (at least) HiPath 3000 V9 (so changing Hardware and Software, worrying also about licensing if you have ISDN PRI Lines) just to have it equipped with an HG1500 V3 (Support of SIP was introduced along the way only with latest HG1500 V3 Software releases)…and all this just to say “WOW, now I’ve finally SIP support on my integrated Siemens Media Gateway!”. IMHO it’s a no go…in term of labour, costs and troubles you’re going to experience…more…thinking that HiPath 3000 was declared Phased Out adds some real blue to all this.
Finding, configuring and using an old Siemens Xpress@LAN is not an option: no SIP Support, just H.323/CorNet-IP (which is proprietary CorNet-N incapsulated into IP, called HFA when used with clients and CorNet-IP when used for IP trunking) …and I don’t know if you would go down that sandy road!
The other way could be using an external SIP to ISDN (TE mode) Media Gateway so you can connect your Hicom 150E R2.2 unused Hicom system ISDN trunks channels (2, 4 or more Channels per Trunk…it depends also on Media Gateway supported ports) and have it registered to your Asterisk+FreePBX system. Results may vary due to Media Gateway configuration/interoperability.
Just my 2 cents (or just 1 Euro cents!).
An advice: Clearly you should provide the topology of your systems/nodes in order to receive good feedback…but I’ve no fear to say that you should throw away that old PBX (indeed it is really rock solid on its basic digital features) and migrate all your nodes to IP each one with/without an integrated ISDN Media Gateway (and, keep in mind, if you have lot of DECT Terminals implemented through Hicom Cordless Office board consider that Gigaset Pro has a DECT/IP solution that integrates flawlessly with Asterisk+FreePBX)…at this point all I wrote is a function of costs your company have to sustain to throw away a (small/medim/large) set of old (end EOL) optiSet E (or old and Phased Out optiPoint 500) digital propietary phones in favour of new SIP IP phones.