HMR to remove crypto offer from SDP

We are trying to enable SRTP for media but running into an issue with older cisco 7940/7960 phones. Whenever SDP contains an offer including crypto, phones respond with "399 Bad Request - 'Invalid SDP information" and do not accept the calls. Is there a way to strip crypto offer from SDP for specific user agents? Basically we need to strip everything below ===== line.

v=0

o=- 14317108 14317108 IN IP4 1.2.3.4

s=Asterisk

c=IN IP4 1.2.3.4

t=0 0

m=audio 49348 RTP/AVP 0 8 18 101

a=rtpmap:0 PCMU/8000

a=rtpmap:8 PCMA/8000

a=rtpmap:18 G729/8000

a=fmtp:18 annexb=no

a=rtpmap:101 telephone-event/8000

a=fmtp:101 0-16

a=ptime:20

a=maxptime:150

a=sendrecv