Trunk context for a double purpose trunk

I’m using a single SIP trunk to a cisco router configured as ISDN gateway on same LAN
On this router a dual FXS card is installed for a pair of analogue phones
I’ve configured these two extension as custom extension with option dial: sip/trunkname/extensionNumber
On cisco a voip dial-peer matches freepbx extension and routes analogue phones calls toward freepbx itself
On cisco also a set of dial-peers matches incoming and outgoing calls from ISDN network to freepbx and vice-versa
When I dial a pbx extension from cisco FXS ports I get back a “the extension is not in use” message unless I declare context=from-internal on SIP trunk
Doing so, ISDN calls are still OK
Is there something wrong in configuring a gateway trunk this way ??