you seem to have the correct format , but note you can specify transport as well… for instance mine looks like sip:[email protected]:5060;transport=tcp
packet capture from here is all you need to see whats actually in the offer , and you should also see if there is an existing redirect via sangoma portal … it will ignore option 66 if one exists
if you are sharing S and P series and want a common option for the bunch use the traditional option string … it will work for both whereas the DPMA option will not work for S series