Why might OpenVPn not work over mobile network?

Question

openvpn

Tell me, please, what could be the problem. There is an OpenVPN server on Debian 8.3 with the config:

port 12345
proto udp
dev tun
ca			ca.crt
cert			server.crt
crl-verify		crl.pem
key			server.key
dh			dh.pem
tls-auth		ta.key 0

client-config-dir ccd
ifconfig-pool-persist 	ipp.txt

server 			1.2.3.0 255.255.255.0
route 			1.2.3.0 255.255.255.0

push 			"dhcp-option DNS 192.168.3.1"
push 			"dhcp-option DNS 8.8.8.8"
push 			"route 192.168.3.0 255.255.255.0"

comp-lzo

user			nobody
group			nogroup
persist-key
persist-tun
status			openvpn-status.log
log			openvpn.log
verb			3
mute			20

Server behind nat. Port forwarded in firewall.
I connect from an android smartphone using OpenVPN Client. If the OpenVPN client establishes a connection via Wifi, there are no problems, but if you try to connect via 3G, then the connection is established, but some applications (in particular CSipSimple) swear "The selected network route is not available with your settings."
If the problem is in the application settings, then I will try to poke around.
In general, is the configuration for openvpn correct? Or something needs to be changed?

Answer 1

Change proto udp to proto tcp

Answer 2

In the default settings of CSipSimple, it is allowed to work only via WiFi. It is necessary to allow work through 3G and other cellular networks separately for incoming and outgoing calls.