Paro is the airport that connects Bhutan with rest of world. The only airline that operate out of here is Druk Air, which is the National Air Carrier if Bhutan. It operates regular flights from Bhutan to Delhi, Kolkata and Gaya. From Paro Airport, Phuntsholing can be reached in 4 hours, via the Phuntsholing-Thimphu highway.
From Kolkata: The Bhutan Government runs a direct service from Kolkata to Phuntsholing, with buses departing Esplanade Bus Station at 7PM on Tuesday, Thursday and Saturday, with the return journey starting from the Phuntsholing office of Bhutan Post at 3PM on Monday, Wednesday and Friday. The journey takes around 18-25 hours (depending on road work) and costs Rs. 350.
From Siliguri: There are two routes from here, one is via Sevak Bridge and the other goes through Alipurduar and Hasimara. The second route is considered more comfortable as the roads are well laid out and paved. The Bhutan Government buses depart from Siliguri Bus Station on Hill Cart Road at 7:30AM and 1:30PM every day. Tickets are purchased in the bus station, but the bus departs from the other side of Hill Cart Road.
The nearest railway station to Phuntsholing is at Hasimara, which is roughly a 30 minutes bus ride. Train services to this station are however, not very frequent.
New Alipurduar railway station is a much better option that reduces your road journey to Phuntsholing. From here, Phuntsholing is only about 60 kilometers and can be reached in a little over an hour by road. Alternatively, there are also buses from New Alipurduar to Phuntsholing are also available.
The town of Phuntsholing is abundant in natural beauty and the best option would be to cover it by foot and enjoy the natural surroundings. Taxis are also widely available for travel within and outside the town. The fare for these taxis is only meter-based.