Clients unreachable

I have an enviremont,
Mobile devices clinet (iOS, Android) on Linphone.

I am observing that the client session times out or is unreachable after the application goes to background/sleep mode or is not being used for some time.
At that time, when the client is reached over sip like …

AGI Script Executing Application: (Dial) Options: (SIP/923124419969,40r)
== Using SIP RTP CoS mark 5

I get …

[Apr 7 17:35:31] WARNING[20329]: chan_sip.c:3661 retrans_pkt: Retransmission timeout reached on transmission 170c252c1e91a5d450b8d9b96689773c@
891 for seqno 102 (Critical Request) – See … nsmissions
Packet timed out after 31999ms with no response
[Apr 7 17:35:31] WARNING[20329]: chan_sip.c:3690 retrans_pkt: Hanging up call 170c252c1e91a5d450b8d9b96689773c@ - no reply to our critical packet
(see … nsmissions).
== Everyone is busy/congested at this time (1:0/0/1)

I cannot use qualify=yes in realtime as it requires rtcachefriend=yes and i cannot use it , not feasible.

Client, not reachable can make calls successfully, but does not get back

Please suggest any solution!