receiving random calls from unknown numbers

[newpond]

hi I seem to be receiving calls from random numbers. is there a way to change the listen option to reject rather than listen or accept?
I dont need incoming calls on this line, it needs to make outgoing calls only.

the state on the sip line sensor changes from false to true when this happens, presumably tying up my line

the lines below stating to: address are not my number

Via: SIP/2.0/UDP 0.0.0.0:63413;received=51.91.168.102;branch=z9hG4bK1899629908
Call-ID: 860059141-613124541-1513302258
From: sip:0000091125588:5060@91.125.58.8;tag=1611136148
To: sip:00000972597580097@91.125.58.8;tag=z9hG4bK1899629908
CSeq: 1 INVITE
Warning: 399 c7744bff-ha-sip "Missing SDP rtpmap for dynamic payload type (PJMEDIA_SDP_EMISSINGRTPMAP)"
Content-Length: 0
--end msg--
15:58:09.289 pjsua_core.c .TX 407 bytes Response msg 400/INVITE/cseq=1 (tdta0x3c2792d0) to UDP 51.91.168.102:63413:
SIP/2.0 400 Bad SDP
Via: SIP/2.0/UDP 0.0.0.0:63413;received=51.91.168.102;branch=z9hG4bK1899629908
Call-ID: 860059141-613124541-1513302258
From: sip:0000091125588:5060@91.125.58.8;tag=1611136148
To: sip:00000972597580097@91.125.58.8;tag=z9hG4bK1899629908
CSeq: 1 INVITE
Warning: 399 c7744bff-ha-sip "Missing SDP rtpmap for dynamic payload type (PJMEDIA_SDP_EMISSINGRTPMAP)"
Content-Length: 0

thanks
Jon