[asterisk-users] T38 faxing - UDPTL creation failed

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Hello guys.

When I am trying to send fax through T38 to linksys SPA (properly
configured etc. - I have tried it with other systems), I'm getting error
and fax is not delivered.

I'm getting this errors in asterisk.log:
WARNING[687] udptl.c: No UDPTL ports remaining
ERROR[687] chan_sip.c: UDPTL creation failed
WARNING[687] udptl.c: No UDPTL ports remaining

then, couple lines down:
WARNING[3514] chan_sip.c: Unsupported SDP media type in offer: image 16400
udptl t38
WARNING[3514] chan_sip.c: Failing due to no acceptable offer found

sip_general_custom.conf contains t38pt_udptl=yes

udptl.conf contains:
[general]
udptlstart=4000
udptlend=4999
T38FaxUdpEC = t38UDPRedundancy

Asterisk version is 1.8.5.0

When I restart asterisk, everything is working good. Then, after some
time, fax stop working.
Do you have any idea what it could be?

Thanks in advance.