The user is not added using the ul_insert_ucontact() function :

ul_insert_ucontact("sip:900@192.168.51.138:1234", "<sip:900@192.168.51.138:1234>", "303", -1, "a2ecd821d4464d75851b73c35442b6ad", 2222, "0", "1", "MicroSIP/3.20.6", "udp:192.168.51.106:5060");

And the following error is displayed in the syslog: 

Sep 18 14:36:48 srv139937 kamailio: CRITICAL: <core> [core/cfg.y:3686]: yyerror_at(): parse error in config file /usr/local/etc/kamailio//kamailio.cfg, line 838, column 148: Too many arguments#012

Sep 18 14:36:48 srv139937 kamailio: CRITICAL: <core> [core/cfg.y:3686]: yyerror_at(): parse error in config file /usr/local/etc/kamailio//kamailio.cfg, line 838, column 148: '('')' expected (function call)

Sep 18 14:36:48 srv139937 kamailio: CRITICAL: <core> [core/cfg.y:3686]: yyerror_at(): parse error in config file /usr/local/etc/kamailio//kamailio.cfg, line 838, column 148: bad command: missing ';'?

Sep 18 14:36:48 srv139937 kamailio: CRITICAL: <core> [core/cfg.y:3683]: yyerror_at(): parse error in config file /usr/local/etc/kamailio//kamailio.cfg, line 838, column 150-152: bad command

Sep 18 14:36:48 srv139937 kamailio: CRITICAL: <core> [core/cfg.y:3686]: yyerror_at(): parse error in config file /usr/local/etc/kamailio//kamailio.cfg, line 838, column 153: bad command

Sep 18 14:36:48 srv139937 kamailio: CRITICAL: <core> [core/cfg.y:3683]: yyerror_at(): parse error in config file /usr/local/etc/kamailio//kamailio.cfg, line 838, column 155-171: bad command

Sep 18 14:36:48 srv139937 kamailio: CRITICAL: <core> [core/cfg.y:3686]: yyerror_at(): parse error in config file /usr/local/etc/kamailio//kamailio.cfg, line 838, column 172: bad command

Sep 18 14:36:48 srv139937 kamailio: CRITICAL: <core> [core/cfg.y:3683]: yyerror_at(): parse error in config file /usr/local/etc/kamailio//kamailio.cfg, line 838, column 174-198: bad command

Sep 18 14:36:48 srv139937 kamailio: CRITICAL: <core> [core/cfg.y:3686]: yyerror_at(): parse error in config file /usr/local/etc/kamailio//kamailio.cfg, line 838, column 199: bad command

Sep 18 14:36:48 srv139937 kamailio: ERROR: <core> [core/cfg.y:3532]: yyparse(): function used inside params of another function: is_method

what shoud i do?