t1-in-millis

500

(optional) Specifies the duration of the SIP timer T1 (round trip time estimate) in milliseconds. For unreliable transports, such as UDP, the client transaction retransmits requests at an interval that starts at T1 seconds and doubles after every retransmission. T1 is an estimate of the round-trip time (RTT). Nearly all of the SIP transaction timers scale with T1, and changing T1 adjusts their values. 

t2-in-millis

4000

(optional) Specifies the duration of the SIP timer T2 (maximum retransmit interval for non-INVITE requests and INVITE responses) in milliseconds. For unreliable transports, requests are retransmitted at an interval which starts at T1 and doubles until it reaches T2. If a provisional response is received, retransmissions continue for unreliable transports, but at an interval of T2. T2 represents the amount of time a non-INVITE server transaction takes to respond to a request if it does not respond immediately. 

t4-in-millis

5000

(optional) Specifies the duration of the SIP timer T4 in milliseconds. T4 represents the amount of time the network takes to clear messages between client and server transactions.