1(military) line of troops encircling an enemy position
2(telephony) outside line
3(basketball) outside the three-point line
Example
wǒ我kě yǐ可以yòng用zhè这bù部diàn huà电话bō dǎ拨打wài xiàn外线diàn huà电话ma吗？
我可以用这部电话拨打外线电话吗？
Can I make an outside call on this phone?